The present invention relates to thermostats and other thermal comfort controllers. The present invention particularly relates to controlling the operation of air conditioners. The invention is typically implemented in a mechanical cooling device that uses a microcontroller in conjunction with temperature sensors.
Thermostats in use now typically direct the operation of air conditioners using dry bulb temperature as the control variable. A typical controller in air conditioning mode causes the air conditioning to begin operation when the temperature rises above the set point value. The air conditioner responds by injecting cold air into the enclosure until the temperature within the enclosure has fallen to a point below the set point value. A typical thermostat uses an anticipation element so as to turn on the air conditioning before the actual set point is exceeded. For many situations this type of control results in air temperature which is comfortable for the enclosure""s occupants. It is well known that an air conditioner removes humidity from the air as well as cools it. The mechanism by which humidity is removed involves passing air from the enclosure or from the outside through the air conditioner, reducing the temperature of this air to substantially less than the comfort range (e.g. less than 74 degrees Fahrenheit).
In order to remove humidity from the air, the temperature of at least some of the cooled air must be lowered to less than the current dew point temperature, the temperature at which water condenses from the air. In this process, some of the water in the air condenses on the cooling coils of the air conditioner and drips off the coils to a pan below. Because air will not release any of its humidity until it reaches 100% relative humidity, i. e. its dew point temperature, it is necessary for at least the air adjacent to the cooled surfaces of the heat exchanger to reach this temperature. The total air stream through the air conditioner, however, may not reach 100% relative humidity because not all the air is cooled to its dew point. Consequently, relatively cold and dry air conditioned air is mixed with the uncomfortably warm and humid air to achieve a more acceptable 40-60% relative humidity at a comfortable temperature of 70-75 degrees Fahrenheit.
Normally such a procedure results in air within the enclosure whose humidity is within the desired comfort range. However, there are situations where humidity levels are still too high, resulting in an uncomfortable enclosure even when the temperature requirement has been met. To achieve air at the comfortable levels of both temperature and humidity, an air conditioner is sized for the expected load which the enclosure will present so that when the set point temperature is reached, humidity is acceptable. In cases of unusually high humidity or the air conditioner capacity relative to the current environmental conditions does not result in dehumidification when the set point temperature is reached, it is possible for the air in the enclosure to have excessive humidity.
Previous attempts to control the relative humidity in enclosures have been made by simply adding a relative humidity sensor to the thermostat and then controlling the air conditioner to hold the relative humidity within the selected set point range. The problem with these approaches is that the relative humidity of the enclosure air may actually rise as the air is cooled and dehumidified within the enclosure. This is because the relative humidity is a function of both the amount of water vapor in a given volume or mass of air and its dry bulb temperature. Relative humidity for any volume of air is defined as the ratio of the partial pressure of the water vapor in the air to the vapor pressure of saturated steam at that temperature. Since the vapor pressure of saturated steam drops rapidly within a temperature, a relatively small amount of water vapor and volume of air at a lower temperature can result in 100% relative humidity. Thus it is possible to have a run-a-way situation where the humidity control function in a thermostat continues to call for further dehumidification, and as the temperature within the enclosure falls, relative humidity rises and locks the air conditioning on.
Subsequent attempts to solve the problem of high humidity have involved controlling the dew point temperature of enclosure air independently of the dry bulb temperature. See U.S. Pat. No. 4,105,063 to Bergt and U.S. Pat. No. 4,889,280 to Grald and MacArthur. However, these devices suffer from disadvantages of the achieved enclosure temperature not always being comfortable, and having a potential for over-cycling of the cooling system. Additionally, none of the references listed above provide dehumidification after the dry-bulb temperature set point has been achieved.
Other climate control systems have included using a humidity sensor, and a dry bulb temperature sensor in the enclosure. See U.S. Pat. Nos. 5,737,934 and 5,675,979. Control of humidity using a reheat system which re-heats chilled air in order to keep the dry bulb temperature of an enclosure to a specific set point is disclosed in U.S. Pat. No. 6,012,296. Another invention on the subject of temperature and humidity control has emphasized using the numerically larger of the dry bulb and humidity temperature errors. An indoor climate controller system adjusting both dry-bulb temperature and wet-bulb or dew point temperature in an enclosure is disclosed in U.S. Pat. No. 5,346,129 and is incorporated herein by reference.
In view of the above, it is apparent that there is a need to provide a more reliable and efficient system for controlling a climate modifying apparatus such as an air conditioner, heat pump, fan coil unit, and the like, when there is unusually high humidity. There is also a need to provide a climate control device that does not over cycle or leave the air conditioning stuck in the ON position.
In view of the foregoing, it is an object of the present invention to provide control of an indoor climate modifying apparatus such as an air conditioning unit to maintain thermal comfort for the occupants of enclosures. Another object is to control the operation of a mechanical cooling device such as an air conditioner, heat pump operating in the cooling mode, fan coil unit operating in the cooling mode, or the like.
A further object of the invention is to have the dry bulb temperature and the humidity temperature error continuously monitored.
Still another object of the invention is to provide a memory that records the dry bulb temperature set point value and a humidity temperature set point value, providing a set point signal encoding the dry bulb and humidity temperatures set point values.
A further object of the invention is to provide a humidity temperature value that is used in connection with the dry-bulb temperature to generate an error signal that is a function of either the dry-bulb or the humidity temperature values. This permits control of both enclosure temperature and enclosure humidity without abnormal cycling of the climate control system.
A further object of the invention is to provide error values for input into a temperature control algorithm used by a controller of the climate control system to determine the times to activate the climate control system for modifying the temperature and humidity of air within the enclosure.
In one embodiment of the present invention, a controller continuously monitors the dry bulb temperature error and the humidity temperature error within the enclosure and controls the ON/OFF status of the cooling device based on the following criteria: a) if the humidity temperature error is less than or equal to zero, the dry bulb temperature error is used in a conventional PID (proportional, integral, derivative) control block to control the ON/OFF status of the cooling device, modifying the enclosure temperature and humidity; or b) if the humidity temperature error is greater than zero, the dry bulb temperature error is ignored regardless of its magnitude and the humidity temperature error is used in a conventional PID control block to control the ON/OFF status of the cooling device; or c) if both the humidity temperature error and the dry-bulb temperature error are less than zero, the numerically larger of the humidity temperature error and the dry-bulb temperature error is used in a conventional PID control block to control the ON/OFF status of the cooling device. Both the humidity temperature error and the dry bulb temperature error use the same PID control block and controller gains to prevent any sporadic equipment operation.
These and other objects not specifically enumerated herein are believed to be addressed by the present invention which contemplates a controller for a climate control system that can give priority to humidity temperature control.